1. Field of the Invention
The present invention relates to an insulated synchronous rectification DC/DC converter.
2. Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., each operate receiving commercial AC electric power from an external circuit. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet terminals are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (AC/DC converter) that performs AC/DC conversion of commercial AC voltage. Alternatively, in some cases, such an AC/DC converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing a basic configuration of an AC/DC converter 100r investigated by the present inventor. The AC/DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 200r. 
The commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise included in the commercial AC voltage VAC. The rectifier circuit 104 is configured as a diode bridge circuit which performs full-wave rectification of the commercial AC voltage VAC. The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106, thereby generating a converted DC voltage VIN.
An insulated DC/DC converter 200r receives the DC voltage VIN via an input terminal P1, steps down the DC voltage VIN thus received so as to generate an output voltage VOUT stabilized to a target value, and supplies the output voltage VOUT thus stabilized to a load (not shown) connected between an output terminal P2 and a ground terminal P3.
The DC/DC converter 200r includes a primary-side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, a secondary-side controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C2, a switching transistor M1, and a synchronous rectification transistor M2. The output circuit 210 has the same topology as those of typical synchronous rectification flyback converters, and accordingly description thereof will be omitted.
The switching transistor M1 connected to the primary winding W1 of the transformer T1 performs switching so as to step down the input voltage VIN, thereby generating the output voltage VOUT. With such an arrangement, the primary-side controller 202 adjusts the duty ratio of the switching of the switching transistor M1.
The output voltage VOUT of the DC/DC converter 200r is divided by means of resistors R1 and R2. The cathode (K) terminal of the shunt regulator 206 is connected to a light-emitting element (light-emitting diode) on the input side of the photocoupler 204. The anode (A) terminal of the shunt regulator 206 is grounded. The divided voltage (voltage detection signal) VOUT_S is input to a reference (REF) terminal of the shunt regulator 206. The shunt regulator 206 includes an error amplifier that amplifies the difference between the voltage detection signal VOUT_S and a reference voltage VREF (not shown) so as to generate an error current IERR that corresponds to the difference, which is drawn (as a sink current) via the light-emitting element (light-emitting diode) on the input side of the photocoupler 204.
A feedback current IFB flows through a light-receiving element (phototransistor) on the output side of the photocoupler 204 according to the error current IERR that flows on the secondary side. The feedback current IFB is smoothed by means of a resistor and a capacitor, and is input to a feedback (FB) terminal of the primary-side controller 202. The primary-side controller 202 adjusts the duty ratio of the switching transistor M1 based on the voltage (feedback voltage) VFB at the FB terminal.
The secondary-side controller 300r switches on and off the synchronous rectification transistor M2 in synchronization with the switching of the switching transistor M1. The secondary-side controller 300r includes a synchronous rectification controller and a driver. The synchronous rectification controller generates a pulse signal in synchronization with the switching of the switching transistor M1. For example, when the switching transistor M1 turns off, the synchronous rectification controller sets the pulse signal to a first state (e.g., high level) configured as an instruction to turn on the synchronous rectification transistor M2. When a secondary-side current IS that flows through the secondary winding W2 becomes substantially zero in an on period of the synchronous rectification transistor M2, the synchronous rectification controller sets the pulse signal to a second state (low level) configured as an instruction to turn off the synchronous rectification transistor M2. The driver switches on and off the synchronous rectification transistor M2 according to this pulse signal.
In order to turn on the synchronous rectification transistor M2, there is a need to apply a gate voltage to the gate of the synchronous rectification transistor M2 that is higher than its source voltage VS by a predetermined voltage. In FIG. 1, the synchronous rectification transistor M2 is arranged on the high electric potential side of the secondary winding W2, i.e., on the output terminal P2 side. With such an arrangement, the source voltage VS of the synchronous rectification transistor M2 changes according to the switching of the switching transistor M1. With such a topology, in order to provide the switching of the synchronous rectification transistor M2, the ground (GND) terminal of the secondary-side controller 300r is connected to the source of the synchronous rectification transistor M2. With such an arrangement, the secondary-side controller 300r is required to operate with the source voltage VS as the reference voltage.
Furthermore, the secondary-side controller 300r is required to receive, via its power supply (VCC) terminal, a power supply voltage VCC1 generated with the source voltage VS of the synchronous rectification transistor M2 as the reference voltage. In order to generate the power supply voltage VCC1, an auxiliary winding W4 is arranged on the secondary side of the transformer T1. The auxiliary winding W4, a diode D4, and a capacitor C4 form an auxiliary converter, which generates the DC voltage VCC1 that is higher than the output voltage VOUT. That is to say, the DC/DC converter 200r shown in FIG. 1 requires such a transformer T1 including such an auxiliary winding W4. However, such a transformer T1 is a high-cost component.